The present invention relates generally to scanning electron beam computed tomography X-ray systems, and more particularly to eliminating cone beam error in images produced by such systems, and thus the need to correct for such error, and to providing such systems with an option to make multi-slice scanning of moving objects without requiring that acquired data be interpolated.
A century ago, mathematician J. Radon demonstrated that a two-dimensional slice of a three-dimensional object may be reproduced from the set of all of its projections. Computed tomography (CT) X-ray systems generate a set of X-ray beam projections through an object to be examined. The resultant detected X-ray data are computer processed to reconstruct a tomographic image-slice of the object.
Conventional CT systems subject the object under examination to one or more pencil-like X-ray beams from all possible directions in a plane. The X-ray data may be generated in fan beam format (as is the case for the present invention), or in parallel beam format. In a fan beam system, the X-rays radiate from a source and are collected in a fan. By contrast, in a parallel beam system the X-rays are all parallel within a view. In either system, a view is one projection of the object onto the detectors, and a scan is a collection of all of the views.
In a fan beam scanning electron beam system such as described in U.S. Pat. No. 4,521,900 to Rand, or U.S. Pat. No. 4,352,021 to Boyd, an electron beam is produced by an electron gun and is accelerated downstream along the z-axis of an evacuated chamber. Further downstream a beam optical system deflects the electron beam about 30xc2x0 into a scanning path, with azimuthal range typically about 210xc2x0. The deflected beam is then focused upon a suitable target, typically a large arc of tungsten material, which produces a fan beam of X-rays.
The emitted X-rays penetrate an object (e.g., a patient) that is disposed along the z-axis and lying within a so-called reconstruction circle. X-ray beams passing through the object are attenuated by various amounts, depending upon the nature of the object traversed (e.g., bone, tissue, metal). One or more X-ray detectors, disposed on the far side of the object, receive these beams and provide signals proportional to the strength of the incoming X-rays.
Typically the output data from the detectors are processed using a filtered back-projection algorithm. Detector data representing the object scanned from many directions are arranged to produce image profiles for each scan direction. Since the X-rayed object is not homogeneous, these profiles will vary in intensity with the amount of radiation detected by the various detectors on the various scans. The convoluted data from the various projections are then superimposed, or back-projected, to produce a computed tomographic image of the original object. The thus processed data are used to produce a reconstructed image of a slice of the object, which image may be displayed on a video monitor.
Systems similar to what is described in the above patents to Rand or Boyd are manufactured by Imatron, Inc., located in South San Francisco, Calif. These systems are termed xe2x80x9cshort scanxe2x80x9d because the views used for reconstructing an object image cover 180xc2x0 plus the fan beam angle (about 30xc2x0), e.g., about 210xc2x0 total, rather than a full 360xc2x0. In a scanning electron beam CT system, the 210xc2x0 angle implies that the target and detector must overlap, which is to say occupy the same space azimuthally.
In prior art systems this problem has been solved by using a parallel planar target and detector that are separated axially. In such systems, the X-ray detectors also span 180xc2x0 plus the fan angle, and define a first plane that is orthogonal to the z-axis. The source of the X-rays scans or travels within a second plane, also orthogonal to the z-axis, but not coincident with the first plane. As the X-ray fan rotates around the target, the central ray from the target to the detector describes a shallow cone rather than an ideal plane. Thus, although ideally reconstruction creates an image in a plane perpendicular to the z-axis using views acquired within that plane, in practice the presence of a cone angle in prior art systems results in each acquired view being inclined rather than perpendicular to the z-axis. This xe2x80x9ccone beamxe2x80x9d effect causes data sets acquired from axially non-invariant objects to be self-inconsistent, which results in image artifacts that degrade image quality and can produce false diagnoses in medical applications.
Unless the cone beam geometry is accounted for by using data from more than one axial position, cone beam error results. The result is a reconstructed image that includes unwanted cone beam artifacts that appear as streaks in the reconstructed, displayed image. In general, cone beam artifacts can be reduced in part only at the expense of scanning contiguous or overlapping slices, and interpolating the data from adjacent slices. U.S. Pat. No. 5,406,479 (1995) to Harman, assigned to Imatron, Inc., assignee herein, describes a method of reconstructing data acquired from a fan beam system such that cone beam error is substantially reduced. However efficient as the Harman technique is, it still requires data processing steps that would not be required if cone beam error could simply be eliminated as an error source. Applicants refer to and incorporate by reference U.S. Pat. No. 5,406,479 to Harman, U.S. Pat. No. 4,521,900 to Rand, and U.S. Pat. No. 4,352,021 to Boyd.
Another problem associated with prior art scanning electron beam CT systems occurs in so-called helical or spiral scanning. In this mode of operation, an object is continuously scanned while being moved at a constant velocity in the axial direction. During scanning, azimuthal and axial motions of the X-ray fan become mixed such that further interpolations must be performed on the resultant data before planar images can be reconstructed. The requirement to perform data reconstruction slows down the reconstruction process, reduces system throughput, and adds to the processing requirements of the overall system.
Thus in a scanning electron beam CT system, there is a need for a method and system to substantially eliminate cone beam error, and thus a need to compensate or correct for such error. Preferably the resultant system should exhibit higher quality images with reduced false diagnoses in medical applications. Finally, the reduction in data processing realizable by such systems should make possible scanning of continuously moving objects with image reconstruction substantially in real-time.
The present invention provides such a system.
A scanning electron beam computed tomographic system eliminates axial offset between target and detector, and resultant cone beam error, by disposing the target, detector, and collimator elements such that active regions of the target and detector are always diametrically opposite each other. This result is obtained by using a helical target, helical collimator, and helical detector. Alternatively, the target, collimator, and detector may be planar, but tilted or inclined about the vertical axis or other transverse axis to approximate an ideal helical configuration.
Eliminating the need to correct for cone beam error and, in some modes of operation, eliminating the need to interpolate data, reduces system cost, reduces computational overhead, and can increase system throughput. Higher quality images are produced, and the decreased data processing makes possible scanning of continuously moving objects and reconstructing acquired images substantially in real-time.
In the helical target, detector, collimator embodiment, the pitch of the helices is sufficient to separate the target and detector axially in overlap regions. The electron beam spot is scanned around the helical target at constant angular velocity to produce an X-ray fan whose axial position moves at a constant velocity Vcrit along the scanner axis.
In a first, helical, mode of operation, the object being scanned is moved at this same, critical, constant velocity Vcrit such that the object slice being scanned remains fixed with respect to the object. In this mode, discrete planar slices are scanned, without cone beam error and without need for data interpolation. A second mode may be used for objects that are moved at non-critical velocity. In this mode helical scanning similar to what is practiced with conventional scanning electron beam CT systems is possible. Data interpolation is needed to produce images, but there is no cone beam error, or need for cone beam correction. In a third mode, step mode scanning is used, in which data interpolation is needed, but no cone beam error or cone beam correction is required.
In the helical or tilted embodiments thus far described, detectors in the detector array were disposed in the azimuthal direction, but not the axial direction. In an alternative embodiment, an array of detectors that includes detector elements disposed in the axial (z-axis) direction is disclosed. Such a multiple axial detector array can provide for more efficient use of X-ray dosage for narrow slices, which increases signal/noise ratio and decreases total scan time. Further, the cone beam effect may be made self-cancelling for images obtained from the various detectors.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.